How Accurate Are Turning Volume Counts Collected by Microwave Sensors?

The Utah Department of Transportation has recently developed the Signal Performance Metrics System (SPMS) to evaluate the performance of signalized intersections dynamically. This system currently provides data summaries for several performance measures, one of them being turning movement volume counts. As this system became public, there was a need to evaluate the accuracy of the data collected by microwave sensors for the SPMS. UDOT considered first that sensor position, number of approach lanes, and volume level would affect the accuracy. Later, speed limit was added as another factor. Data were collected manually and compared with volume count data reported by the system. A Mixed Model Analysis of Variance was employed to analyze the effect of each factor on the accuracy of traffic volume counts. The volume level and number of approach lanes factors were found to have a statistically significant effect on the accuracy of traffic volume counts by microwave sensors.